Cast-iron composition of high refractoriness and strength and process for making same



United States Patent 3 Claims. 61. 148-3) ABSTRACT OF THE DISCLOSURE A process for making cast iron of high refractoriness and strength for pistons, piston rings, valves and valve seats, wherein an iron melt which a carbon content between 1.4 and 1.8% and a silicon content between 1.8 and 2.2% by weight is admixed with one or more austenite-stabilizing agents consisting of 0.7 to 1.5% by weight of nickel, 0.3 to 0.6% by Weight of chromium, 0.5 to 0.8% by weight of manganese and 0.1 to 0.2% by weight of vanadium; the melt is cooled at a rate of 250 C./hour to an austenitic-martensitic structure free from primary carbon and ledeburite and with minor proportions of cementite and troostite, and globular secondary graphite is finely distributed throughout the structure by briefly annealing the structure at 800 C. to 1000 C. for a maximum of one hour.

Our present invention relates to cast iron adapted to be used in the manufacture of structural parts that are subject to severe mechanical and thermal stresses, e.g. pistons, piston rings, valve-seat rings and the like.

The general object of this invention is to provide a cast-iron composition of highly refractory character, great wear resistance and large bending strength.

'Another'object of this invention is to provide a process for making cast-iron bodies of this description which are free from holes and other structural defects.

Prior processes for making cast iron were generally based on a control of the primary graphite within the structure, a finer degree of distribution of this graphite being considered desirable. To this end it was, however, necessary to operate under thermal conditions which would result at least in part in a white casting essentially constituted by ledeburite with a proportion of cementite adapted to be dissolved only by a subsequent heat treatment with liberation of secondary graphite. The presence of this white casting involves the danger of hole formation, and furthermore, the advantage of a finely distributed primary graphite is largely offset by the occurrence of coarse secondary graphite upon a prolonged annealing as needed to dissolve the ledeburite.

We have found, in accordance with this invention, that cast-iron compositions of considerable high-temperature strength, hardness, tensile strength and wear resistance, suitable for sand or chill casting to form construction elements of the type mentioned above, can be produced as austenitic-martensitic structures free from ledeburite and primary graphite incorporating a globular carbonaceous decomposition product of martensite, i.e. secondary graphite in finely comminuted form. Such a composition not only satisfies the aforestated desiderata but also exhibits good ductility in the range of both elastic and plastic deformation. A particularly high de gree of wear resistance is observed when the structure includes a distinct phase predominantly of nickel at the grain boundaries.

3,360,407 Patented Dec. 26, 1967 The process according to our invention starts with the production of an iron melt with low C and Si content, specifically with a carbon content between about 1.4 and 1.8% and a silicon content between about 1.8 and 2.2%, by weight. Admixed with this melt are one or more austenite-stabilizing agents such as nickel, chromium, manganese and/or vanadium. Upon controlled cooling of the melt, the latter hardens to an austenitic-martensitic structure free from primary carbon and ledeburite and with minor proportions of cementite and troostite. The carbon dissolved in the martensite is thereupon partly liberated by a brief annealing step, e.g. for about 45 minutes at a temperature of approximately 900 C., with subsequent oven cooling, resulting in the precipitation oi finely divided globular secondary graphite.

The annealing and subsequent oven cooling does not result in the formation of any ferrite and leads directly to the liberation of the comminuted gloublar secondary graphite together with segregation of a nickel-rich phase at the grain boundaries, this phase being particularly noticeable when the duration of the heat treatment is reduced from a preferred maximum of about an hour to a period of approximately 30 minutes. The presence of this distinct phase insures high wear resistance and prevents embrittlement.

A particular advantage of the process according to the invention is that the starting material may be in the form of cheap scrap steel to which merely the aforedescribed alloying admixtures are to be added, the cost of production being further reduced in comparison with conventional processes by the appreciable shortening of the annealing time.

The austenite stabilizers admixed with the basic iron melt may range in proportion from a fraction of a percent (by weight) to several percent cumulatively. More particularly, the weight ranges may be approximately 0.7 to 1.5% for nickel, 0.3 to 0.6% for chromium, 0.5 to 0.8% for manganese and 0.1 to 0.2% for vanadium.

In general, the proportion of these admixtures may be increased when the amount of carbon and/ or silicon is lower, this being advantageously so in the case of castings of larger wall thickness. As a rule, the composition of the melt should be so chosen as to prevent formation of any primary graphite and ledeburite, with suflicient carbon and silicon present to insure the precipitation of the desired secondary graphite. The maximum quantity of austenite stabilizers is determined by the need for a rapid decomposition of the fundamental austeniticmartensitic structure during heating, the minimum quantity of these stabilizers being that which just sufiices to prevent the conversion of this structure into ledeburite.

The cooling of the melt may be carried out continually at a uniform rate, e.g. 250 C. per hour, which directly leads to a transition phase. When the subsequent heat treatment, in the range of about 800-1000 C., is extended to about an hour, a sorbitic structure results with increased elastic limit; this heat treatment may be followed by air cooling to about 600 C.', reheating to about 750 C. and further cooling at a rate of about C. per hour. To promote the precipitation of nickel phase, this heat treatment should be shortened to about 30 minutes.

The composition produced in accordance with this invention has a very good high-temperature strength in a range of about 200500 C., this being believed due to the fact that no ferrite is precipitated even with very slow cooling. After annealing, the material has a Brinell hardness ranging between about 250 and 750 and bending strengths, e.g. as measured in tangential direction on piston rings or the like, on the order of 50 to kp./mm. Maximum bending strength has been found to occur around a Brinell hardness of 330.

to 1000 C. for a maximum time of approximately one hour, the annealing step being followed by air cooling to a temperature of substantially 600 C. and by re- Example Sc ap Steel and Was e castings are t d d n i an 5 heating to substantially 750 C. with subsequent cooling at a rate of about 100 C. per hour.

f gi g i g g g zi g i 2. A process as defined in claim 1 wherein the anneal- 21 1 2 2 wig? P d g i ing step is carried out at temperatures of substantially material is cast into piston rings with an austenitic- 2 2 2 for a of approguiatelg mmutes martensitic structure andsmall proportions of troostite 10 1 Gas Ion as {P 6 y t 6 2 0 and cementite. Their Brinell hardness is approximately calm 1 and chatafztenzed refractormess f 500' strength, an austenitic-martensitlc structure free from pn- The piston rings thus produced may be heat treated in 'y graphite and ledebllrite having distributed therevarious ways with results as shown on the following table: through a globular carbonaceous decomposition product Brinell Bending Treatment Hardness Strength, Ductility Structure (30/25) kp./mm.

1 h. 725-70" 0.; ovenlco ne Moderate gorlfitte V2 h. 900 Oq s n o li g to 600 0.; air cooling =340 110-115 s bit e plus nickel phase.

y 1 900 (3,; o coolin =330 130-135 Perlite, sorbite plus nickel phase.

Upon admixture of about 0.5% of chromium Do =300 =150 Good Perlite,sorbite.

% 11.1000 0.; air cooling to 600 0.; reheating to 750 0.; oven =340 120-140 Moderately elastic... Do.

% l i?!;0 0 0.; cooling at 250 C./ll =400 70 Good Martcnsite plus transition phase.

We claim: of martensite, and a distinct phase predominantly of 1. A process for making cast iron of high refractoriness nickel at the grain boundaries.

and strength, comprising the steps of producing an iron melt with a carbon content between substantially 1.4 and References Clted 1.8% and a silicon content between substantially 1.8 UNITED STATES PATENTS and 2.2% by weight, admixing with said melt a combination of austenite-stabilizing agents consisting of substantially 0.7 to 1.5% by weight of nickel, substantially 2227217 12/1940 Craftg e 148140 0.3 to 0.6% by weight of chromium, substantially 0.5 to 2438267 3/1948 Bouts X 0.8% by weight of manganese and substantially 0.1 to 2883281 4/1959 Jatczek 3 X 0.2% by Weight of vanadium, controlledly cooling at 2901384 8/1959 Saivea 3 a rate of substantially 250 C. per hour the melt to an 3O13911 12/1961 Pera S "a;

austenitic-martensitic structure free from primary carbon S n and ledeburite and with minor proportions of cementite FOREIGN PATENTS and troostite, and liberating globular secondary graphite 1,121,639 1/1962 Germany.

finely distributed throughout the structure by briefly annealing same at a temperature of substantially 800 C.

CHARLES N. LOVELL, Primary Examiner. 

1. A PROCESS FOR MAKING CAST IRON OF HIGH REFRACTORINESS AND STRENGTH, COMPRISING THE STEPS OF PRODUCING AN IRON MELT WITH A CARBON CONTENT BETWEEN SUBSTANTIALLY 1.4 AND 1.8% AND A SILICON CONTENT BETWEEN SUBSTANTIALLY 1.8 AND 2.2% BY WEIGHT, ADMIXING WITH SAID MELT A COMBINATION OF AUSTENITE-STABILIZING AGENTS CONSISTING OF SUBSTANTIALLY 0.7 TO 1.5% BY WEIGHT OF NICKEL, SUBSTANTIALLY 0.3 TO 0.6% BY WEIGHT OF CHROMIUM, SUBSTANTIALLY 0.5 TO 0.8% BY WEIGHT OF MANGANESE AND SUBSTANTIALLY 0.1 TO 0.2% BY WEIGHT OF VANADIUM, CONTROLLEDLY COOLING AT A RATE OF SUBSTANTIALLY 250*C. PER HOUR THE MELT TO AN AUSTENITIC-MARTENSITIC STRUCTURE FREE FROM PRIMARY CARBON AND LEDEBURITE AND WITH MINOR PROPORTIONS OF CEMENTITE AND TROOSTITE, AND LIBERATING GLOBULAR SECONDARY GRAPHITE FINELY DISTRIBUTED THROUGHOUT THE STRUCTURE BY BRIEFLY ANNEALING SAME AT A TEMPERATURE OF SUBSTANTIALLY 800*C. TO 1000*C. FOR A MAXIMUM TIME OF APPROXIMATELY ONE HOUR, THE ANNEALING STEP BEING FOLLOWED BY AIR COOLING TO A TEMPERATURE OF SUBSTANTIALLY 600*C. AND BY REHEATING TO SBUSTANTIALLY 750*C. WITH SUBSEQUENT COOLING AT A RATE OF ABOUT 100**C. PER HOUR. 